Meteorites reveal Earth's core

29 June 2007

A fragment from the Vesta asteroid.

Scientists have used meteorites from Mars and other planetary bodies to help resolve how the Earth's core formed.

Comparing meteorites provided by the Natural History Museum, with terrestrial rocks, scientists discovered that the Earth's metallic iron core, the area that gives the Earth its magnetic field, is very likely to contain small amounts of silicon.

Silicon is the major constituent of the silicate minerals and rocks that build the Earth's crust and underlying mantle. However, there has been no direct evidence that light elements, such as silicon, exist in the Earth's core since the idea was proposed by Harvard geophysicist Francis Birch in 1952.

Silicon isotopes

The international team, lead by Alex Halliday from Oxford University, applied novel analytical techniques and compared the abundance of isotopes of the element silicon in meteorites with those in rocks from Earth.

Isotopes are atoms of the same element that differ slightly in mass. These small mass differences can be analysed using sophisticated mass-spectrometers and allow the scientists to make conclusions as to how rocks formed.

The team looked for silicon's three stable isotopes with the masses 28, 29 and 30, and found that meteorites from Mars and a large asteroid known as Vesta have higher proportions of light silicon with the mass 28, when compared with rocks from the Earth and the Moon.

The team concluded that this small mass difference of silicon was caused during core formation, when silicon was incorporated into the core.

Mass and size

To the team's surprise they found no evidence for such processes on Mars and Vesta, even though they both also have iron cores.

This difference could be explained because the Earth is eight times larger than Mars and would have experienced much higher temperatures and pressures during core formation. Such high pressures and temperatures are required for silicon to be present in the metallic core.

These findings provide new evidence that the Earth's core formed under different conditions from those on Mars and Vesta.

Moon similarities

Interestingly, the silicon composition of the Earth is the same as the moon's. This can't be explained by high pressure and temperatures during moon core formation as it is only about one per cent of the mass of the Earth.

However, it suggests that the Moon inherited the silicon composition of the early Earth, when it collided with a Mars-sized body resulting in the formation of the moon.